Arc plasma device having gas cooled electrodes containing low work function material



H. FRC5HLICH 3,343,027 ARC PLASMA DEVICE HAVING GAS COOLED ELECTRODESSept. 19, 1967 CONTAINING LOW WORK FUNCTION MATERIAL Filed Aug. 5, 19644 Sheets-Sheet 1 FIG. 1

FIG. 3

Sept. 19, 1967 'H. FROHLICH 3,343,027

ARC PLASMA DEVICE HAVING GAS coousn ELECTRODES CONTAINING LOW WORKFUNCTION MATERIAL Filed Aug. 5, 1964 4 Sheets-Sheet 2 FIG. 2

pt-- 1967 H. FROHLICH I 3 ,343 ,027

ARC PLASMA DEVICE HAVING GAS COOLED ELE CONTAINING LOW WORK FUNCTIONMATERI Filed Aug. 5, 1964 4 Sheets-Sheet 3 Fl G. 7

FIG. 4 FIG.6

Sept. 19, 1967 H. FRDHLICH 3,343,027

ARC PLASMA DEVICE HAVING GAS COOLED ELECTRODES CONTAINING LOW WORKFUNCTION MATERIAL Filed Aug. 5, 1964 4 Sheets-Sheet 4 FIG. 8

United States Patent 14 Claims. 61. 313-231 My invention relates to amethod and device for heating gases in a plasma gun.

In a plasma gun a direct-current or alternating-current arc serves toheat and ionize a flow of gases passing through the gun channel, thusproducing a plasma. Such plasma guns are applicable for a great varietyof purposes. For example, they are used in plasma chemistry forreleasing chemical energy, they can be used for deposition welding andalso for cutting. If the plasma is passed through a nozzle, high flowvelocities are attained which can be used for supplying wind channels orreaction thrust for space-vehicle propulsion.

A known design of plasma burners is provided with rodor pin-shapedcathodes at which electrons issue from emissive materials, such asthoriated tungsten, by thermal field emission. During operation of theseburners, the tip of the cathode is heated to such a high temperaturethat a portion of the electrons is emitted thermally and the otherportion is emitted by field emission and hence by the high fieldstrength in the cathode drop of the arc. The heat dissipation from sucha cathode enforces temperature limitations because of the limitedgeometrical dimensions. These cathodes, therefore, can be subjected onlyup to a relatively low current load. If this limit is exceeded,=thecathode will fuse oil, which results in a short useful life of theequipment.

Plasma guns have therefore been developed in which an arc burns betweenannular electrodes and, while being energized by direct current oralternating current, rotates in a transvese magnetic field. In theseplasma guns, the annular electrodes are intensively cooled, for exampleby designing them as water-cooled copper cathodes. This expedientreduces the efiiciencyof the plasma gun. It has also been found that, athigh current intensities, the material will commence to melt in theburning spot of the cathode, so that material vaporizes and leads to areduction in lifetime of the electrodes. It is an object of my inventionto minimize or overcome the difiiculties encountered with plasma guns inwhich an are between annular electrodes rotates in a transverse magneticfield.

More particularly, it is an object of the'invention to enlarge theburning spot of the arc in such a plasma gun so as to reduce the energydensity and thereby promote .the thermal emission while reducing thedanger of melting and vaporization at the spot.

According to the invention, I employ annular electrodes formed of a mainmaterial Whose melting point is above 1200 C. and with an addition ofsubstances whose electron work function is in the range of 0.15 to 0.7of the electron work function of the main material, and I further adjustthe working temperature of the electrodes by maintaining them at a valuebelow the melting point of the electrodes but above 1000 C.

The invention isbased upon the recognition that due to the intensivecooling of the electrode acting as the oathode, the arc must contractstrongly in the region of the cathode drop in order to attain a smallburning spot such a high temperature, within very short intervals oftime, that thermal emission also can take place. In the known electronguns, this phenomenon causes the electrode material, when operating withhigh current intensities and consequently with a high energy density inthe small burning spot, to become heated to the temperatures at whichthe electrode material melts and vaporizes. In contrast thereto, thepresent invention, by virtue of the above-mentioned features, providesfor a large burning spot on the cathode which distributes the energyfurnished through the electrode material and promotes thermal emission.

When performing the method according to the invention and employing thepermissible proper cooling commensurate with the large burning spot, ahigh average electrode temperature is maintained. This results in goodefficiency. Preferably the gas to be heated is employed as the coolingmedium. By the rotation of the arc and the geometry of the electrode,the median energy density can be so limited that even with maximumcurrent intensities the electrodes remain largely unaffected which istantamount to a long useful life.

The above-mentioned main material of the electrodes may consist ofhigh-temperature steel or other refractory metal such as tungsten,molybdenum, tantalum or an alloy, or it may consist of a slntered bodyof refractory metals or alloys. Also suitable are the electro-conductingcompounds of these metals such as the carbides, borides and silicides.The added substances of low work function may consist, for example, ofThO K 0, BaO, SrO, U0 UC, LaB The maximum amount of the added substancesshould be 20% of the mass of the main material.

The addition substance of low work function may be added to the mainmaterial of the electrode by dissolving and/or alloying therein, byadmixture in powdered form to the powdered main material prior topressing and sintering according to the techniques of powder metallurgy,by surface deposition using common metallurgical techniques, bydiffusion, by ceramic surface coating and/or surface inserts at the workface. These expedients for the application of such low work functionadditions to the main material are not critical as long as the choice ofspecific material used as the main material and the additive substanceis made and the combination is operated at the temperatures and in themanner set forth.

The rotating speed of the arc can be adjusted in known manner bycorrespondingly selecting the strength of the magnetic field, thecurrent intensity of the arc, the type of gas or the gas pressure. Themagnetic field can be produced by electromagnets or permanents magnets.For operation with an alternating-current arc, it is preferred that thematerial of both electrodes contain the same constituents. Whenoperating with a direct-current arc, the anode may consist only ofrefractory metals or alloys without the above-mentioned additives of lowwork function. The cooling of the electrodes can be augmented, ifnecessary, by radiation cooling with the aid of vanes on the electrodes.Any other suitable cooling means may also be employed or added.

The above-mentioned and further objects, advantages and features of theinvention, said features being set forth with particularity in theclaims annexed hereto, will be apparent from, and will be mentioned in,the following with reference to embodiments of plasma guns according tothe invention illustrated by way of example on the accompanying drawingsin which:

FIG. 1 shows schematically in axial section the fundamental design of aplasma gun with annular electrodes and the rotating direct-current arc.

FIG. 2 is an axial section of another plasma gun embodying theprinciples of the gun shown in FIG. 1.

FIG. 3 is a cross section along the line III-III in FIG. 2. r

FIG. 4 shows in axial section, a modified electrode applicable in aplasma gun otherwise corresponding to FIG. 2.

FIG. is a cross section through the electrode of FIG. 4 along the lineVV in FIG. 4.

FIG. 6 is an axial section through another electrode applicable in aplasma gun otherwise similar to FIG. 2.

FIG. 7 is a cross section along the line VIII-VIII in FIG. 6.

FIG. 8 shows in axial section another embodiment of the plasma gunaccording to the invention in which the arc burns parallel to therotational axis, in contrast to the preceding embodiments where it burnsperpendicularly to the axis of rotation.

Corresponding components are denoted by the same respective referencecharacters in all illustrations.

According to the schematical illustration in FIG. 1,

'a coil 1 produces a magnetic field whose field lines, indicated bybroken lines 2, extend transversely of the are 10. The magnetic fieldcauses the arc to rotate in an an- 'nular' gap formed between twocylindrical electrodes 4 and 6 which are coaxially mounted incylindrical sleeve 3 insulated for the purpose of electrical and thermalinsulation. The inner electrode 6 has a closed end at the location ofthe arc gap and is coaxially centered and held in the outer electrode 4by means of one or more insulating spaces 5 which are perforated bylarge openings for the passage of gas. It serves as cathode for thedirectcurrent arc. I The outer cylindrical electrode 4 serves as theanode 'for' the direct-current are. It consists of high-temperaturesteel or of any of the other refractory metals, alloys, sinteredmaterials mentioned above, or of the likewise mentioned carbides,borides and silicides of refractory metals. Preferably employed ishigh-temperature, corrosion-resistant steel having a melting point above1200 C., which also constitutes the base material for the cathode 6. Thecathode material, however, contains an addition of substances whose workfunction is 0.15 to 0 .7 of that of the base material. As mentioned, theaddition amounts to no more than 20% of the mass of the base materialand consists for example of T110 or the other substances mentioned inthe foregoing. Suitable as base material for the cathode are also any ofthe other materials applicable for the anode.

At any moment, the burning are extends substantially in a radialdirection within the annular gap between the cathodes 6 and the anode 4.At the closed end near the arc gap, the cylindrical cathode 6 isperforated by circular rows of holes 7 for the passage of .gas. Thecylindrical hollow space within the cathode 6 and the cylindricallyannular space between the cathode 6 and the anode 4 constitute coolingchannels. These are traversed by the gas which is to be heated by thearc 10 and which is supplied through the inlet opening at 11. The gasflow is schematically indicated by arrows 12. As the gas passes alongthe electrode walls, each electrode is individually cooled. By adjustingthe gas volume throughput, the working temperature of each electrode canbe adjusted to a value independent of that of the other electrode. Foradditional cooling, helical coils or cooling tubes 8 may be provided forcoolants such as water.

Due to the heat exchange, the gas is heated as it passes through theannulus of the electrodes until it reaches the annular gap and therotating arc. By properly adjusting the cooling expedients, the mediantemperature of the electrodes in the region of the arc can be readilyadjusted to a value below the melting point of the electrodes and above1000 C., thus securing a high efficiency of the plasma gun. The arc 10rotating in the electrode gap about the axis 9 converts the preheatedgas to the ionized condition, so that a flow of plasma issues from thegun. For this purpose, the arc is energized by connecting the anode 4 tothe positive pole of a direct-current supply,

and the cathode 6 to the negative pole by any suitable electrical means,such as the terminal and cable connection shown in FIG. 2.

For operating a plasma gun with a direct-current are it sutfices to addthe above-mentioned additives of low work function only to the basematerial of the cathode. However, such substances are added to the basematerial of both electrodes if the gun is to operate with alternatingcurrent.

The plasma gun according to FIGS. 2 and 3 comprises a cylindrical andcup-shaped inner electrode 6 in coaxial relation to a cylindrical outerelectrode 4. The electrodes consist of nonmagnetic materials inaccordance with the foregoing explanations. The electrode 6 has aflow-dividing internal profile at 13. The electrode 6 is held togetherwith a cylindrical gas-guiding structure 14 by means of copper centeringrods 5. The cylindrical guide structure 14 has an outwardly protrudingflange portion by means of which it is inserted into a recess of aring-shaped intermediate body 15. The body 15 consists of electricallyinsulating, heat-resistant material such as a putty-like mass ofzirconium oxide or boron nitride press-molded to the proper shape, orthe body may also be pressed and molded from Blakite putty. The annularinterspaces between the intermediate body 15 and the outer electrode 4on the one hand, and between the electrode 6 and the body 15 as well asthe guide structure 14, on the other hand, provide for hollowcylindrical cooling channels. The flow cross-sectional area of thesecooling channels may have the same dimension. For this purpose, thewidth of the cooling channels having a larger diameter is smaller thanthe width of the cooling channels having a smaller diameter.

The outerelectrode 4 is mounted by a tight fit or force fit in a tubularhousing 16 of brass or nonmagnetic steel (such as available in the tradeunder the name VZA-steel). The two electrodes 4 and 6 form between eachother an annular gap in which, during operation of the gun, the are 10is maintained while rotating about the axis 9. A heat shield consistingof a cylindrical body 3 of thermally and electrically insulatingmaterial such as boron nitride is mounted on the inner wall of thetubular housing 16 at a location where this heat shield protects thehousing 16 from being impaired by the arc. As shown, an air gap mayremain between the cylindrical heat shield 3 and the electrode 4.

The magnet coil 1 for rotating the arc is wound upon a carrier 17 ofbrass which is coaxially mounted upon the cylindrical housing 16.

The housing 16 is closed by a circular plate 18 of structural steel orbrass. The plate 18 carries a terminal 19 for attachment of a coppertape or cable 20 by means of which the inner electrode is connected toone pole of current source for the arc. The outer electrode 4 isconnected to the other pole of the same source through the cylindricalhousing 16.

The inner electrode 6 is held in concentric relation to the rotationalaxis 9 of the plasma gun by the centering action of the rods 5 whichabut against the terminal 19 and thus are held in proper position by theclosure plate 18. Interposed between the terminal 19 and the closureplate 18 is an electrically insulating seal 21 consisting for example ofpolytetrafluoroethylene. Further sealing members or gaskets, which mayconsist of the same material, are denoted by 21.

A gas collector nozzle 22 which simultaneously mixes the heated andionized working gas, is fitted into the thermally and electricallyinsulating heat shield 3. The nozzle 22 comprises a jacket 23 and aflow-guiding core 24 and may be cooled in its interior by circulatingwater. Due to the provision of an adjacent insulating gasket 21 and theinsulating heat shield 3, the collector nozzle 22 is isolated from theelectrical potentials of the electrodes 4 and 6. This prevents the arcfrom finding a foot point on the nozzle jacket 23.

A mounting ring 25 is gas-tightly fastened to the housing 16 and isjoined with the collecting nozzle 22 by means of screw bolts 26. Theclosure plate 18 on the opposite end of the housing 16 is likewisefastened to the housing by bolts 26. The working gas whichsimultaneously serves as a coolant for the electrodes 4 and 6, issupplied through inlet ducts 11 which are distributed about theperiphery of the housing 16 and extend tangentially thereto (FIGS. 2,3), the flow direction of the gas being indicated by arrows 12. Theworking gas contacts during its flow the inner and outer cylinder wallof the electrode 6 as well as the inner wall of the electrode 4. Thusthe electrodes 4 and 6 are cooled by separate gas flows.

Relative to the dimensions of the wall cross section and axial length ofthe cylindrical electrodes 4 and 6, as well as the cross sections of thecylindrical cooling channels adjacent to the electrodes, the followingconsiderations may serve as a guide. In the first place, the wall crosssection is essential to heat dissipation with respect to cylinderssupplied with heat at an axial end thereof. Also essential in thisrespect is the area of the cooled wall surface. It is to be noted that aprolongation of the cylinder is not as effective as an increase incylinder diameter because the relatively poor heat conductance throughthe electrode walls, since they consist of the base material such astungsten, causes the occurrence of a temperature gradient whosemagnitude cannot be neglected. Accordingly, approximately two thirds ofthe heat quantity generated by the arc must be dissipated on the lastthird of the electrode cylinder length adjacent to the arc. A suitabledimensioning will be further elucidated with reference to an example.Assume that a plasma gun serves to heat a working gas to a temperatureof 3000 K. (about 2700 C.) at an efiiciency of 80%, operating with anare having an output power of l megawatt and a voltage of 500 volts at2000 amps, the voltage drop per electrode being 10 volts. Under theseconditions, 800 kilowatts will heat the working gas to the desiredtemperature. Accordingly, an quantitative gas flow of 225 g. gas persecond should be provided, corresponding to approximately 112 g. persecond per electrode.

The proper dimensioning of the electrodes can be arrived at byconsidering a simplified model. Used is a cylinder of tungsten having aninner diameter of 80 mm. a wall thickness of 3.6 mm. and a length of 80mm. The cylinder is heated along a generatrix. If one assumes that thesource of heat is an electric arc corresponding to the above-mentioneddata, then for an electrode drop of 10 volt per electrode, it isnecessary to dissipate 20 kilowatt by cooling. The heat conductance ofthe tungsten cylinder may be set approximately to be 90 kcal./

m h. C., wherein kcal.=kilocalories, m=length of the cylinder in meters,h.=hours. The heat transfer between electrode cylinder and cooling gasmay be set as 1000 kcal/m h. C., wherein m =area of the electrodecylinder. This corresponds to a mathematical product of flow speed'and'specific gravity of the coolant gas of approximately 300 kg./sec./cm. Itfollows that when the gas is supplied under 'a superatmospheric pressureof 1,

10 or 100 atmospheres, it must flow at respective speeds of 158, 32 and4.3 m./sec. The median cooling-gas temperature at a mass flow of 112g./sec. then amounts to 100 C. With a temperature gradient along thecylinder of 3400 to 3000 C., 16,300 kcal./h.=l9 kw. are then dissipatedto the coolant gas. The cylinder area under these conditions and forthe, above-assumed cylinder dimensions amounts to cm.

On the basis of these reference data for the electrodes, the dimensionsrequired for the cooling channels of the plasma gun can be determined.For the required gas flow of I12 g./sec. for each electrode, and withthe above-mentioned values for the heat transfer and for the product offlow speeds and specific gravity of the coolant gas, there results anarea of 3.7 cm. as required for the flow cross section of the annulargaps that constitute the respective cooling channels.

In the embodiment according to FIG. 2, the inner electrode 6 iscontacted on both sides by the gas flow, whereas the outer electrode 4is contacted only on one side because the inner wall surface of theinner electrode 6 is smaller than that of the electrode 4. Consequently,by cooling both sides of the inner electrode 6 an equalization of thecooling effect at both electrodes is obtained.

FIGS. 4 and 5 show a different embodiment of an inner electrode 6'applicable for the inner electrode of a plasma gun as shown in FIG. 2.The electrode 6' according to FIG. 4 has a cylindrical cup shape. Theopen end of the electrode cylinder has a flow-dividing contour 13 oftapering cross section. This causes two separate gas flows to cool theelectrode Walls on both sides. The inner gas flow passes through a rowof holes 7' to merge with the outer gas flow. The annular coolingchannels are formed between a single centering body 5, the cylindricalportion of the electrode 6' and an intermediate body 15' of which only aportion is illustrated in FIGS. 4 and 5.

FIGS. 6 and 7 show another embodiment of the inner electrode 6 with asingle centering body 5" and a partially illustrated cylindricalintermediate body 15". The electrode 6" has a cylindrical body which atthe upstream side of the gas flow, indicated by arrows 12, is roundedfrom the outer periphery to the inner periphery, and whose other endforms a bulge protruding radially and peripherally beyond thecylindrical main portion. In this embodiment only the outer surface ofthe electrode 6" is cooled by the gas flow. For that reason, thissurface is enlarged by blunt cooling ribs 27 which extend in thelongitudinal direction and are uniformly distributed over the periphery.

In the plasma gun according to FIG. 8, the are 10 extends generally in adirection parallel to the rotational axis 9. The two annular electrodes4a and 4b are axially spaced from each other to form the arc gap betweeneach other so that the travel path of the rotating arc extendsperipherally along the ends of the respective cylindrical electrodes.

Each electrode has the shape of a cuff whose bight portion is adjacentto the arc gap and whose outer portion is doubled back with respect toan inner cylindrical portion. If desired, the electrodes may each becomposed of several parts, particularly in the arc-gap region.

the direction of field lines schematically represented at 1 2, isproduced by magnet coils 1a and 1b. The flow of gas is heated by therotating arc and leaves the plasma gun through a tube 28 located insidethe electrode 4b.

The interior of the tube 28 is subdivided by a cylindrical partition topermit being cooled by a circulating coolant such as water. However theelectrodes proper are cooled only by the working gas which, for thispurpose, can be controlled as to flow quantity as explained heretofore.The two electrodes 4a and 4b are kept in proper positions by means of aspacer body 5 of electrically insulating and heat-resistant material.Copper rods 29 serve as electric current conductors for the electrodes4a and 4b respectively.

If the plasma gun according to FIG. 8 is mounted in a pressure-resistantcontainer wtih only the tube 28 for issuance of the hot working gas fromthe container, then the windings 1a and 1b may be cooled by having themexposed to the flow of cooling gas. Applicable as cooling gas for thelatter purpose is the working gas, or a different gas with which theworking gas is to be mixed and which is supplied to the arc throughannular gaps 30. By means of such mixing, the are energy can becontrolled, and the temperature and properties of the working gas canalso be varied. If desired in the control of the plasma generation, themixing of gases can be altered by closing the annular gaps 30.

It is essential to the invention that by virtue of the cooling appliedin the above-described manner, the electrodes are cooled only moderatelyso that the burning spot of the are on the electrodes is enlarged. Onthe other hand, such enlargement of the burning spot is a prerequisitefor permitting such a mild or moderate cooling because the energydensity for a given arc energy on an electrode is lower in a largeburning spot than in a small burning spot.

To those skilled in the art it will be obvious from a study of thisdisclosure that my invention permits of various modifications and can begiven embodiments other than particularly illustrated and describedherein, without departing from the essential features of my inve'ntionand within the scope of the claims annexed hereto.-

1 claim:

1. A plasma gun, comprising two coaxial annular electrodes spaced fromeach other and forming an annular arc gap between each other, saidelectrodes consisting of an electroconductive refractory main materialhaving a melting point above 1200 C., and at least one of saidelectrodes containing an addition of substance having a work functionwithin 0.15 to 0.7 that of said main material, magnet means havingtransverse field in said annular gap for causing the are between saidelectrodes to rotate along said gap, gas duct means for passing the gasto be ionized through said gap, said duct means comprising coolingchannels along said electrodes for maintaining their working temperatureby heat exchange with substantially all of the gas flow at a value belowthe melting point of the electrodes and above 1000 C.

2. A plasma gun, comprising two cylindrical electrodes coaxially mountedone within the other in radially spaced relation, the inner one of saidelectrodes having one axial end closed and forming with the outerelectrode at said end an annular arc gap coaxial with said electrodesand smaller in radial width than the radial spacing between saidelectrodes along the axial length thereof extending from said end, saidelectrodes consisting of electroconductive refractory material having amelting point above 1200 C. and an addition of substance having a workfunction within 0.15 to 0.7 of the work function of said refractorymaterial, magnet coil means coaxially surrounding said annular arc gapand having therein a transverse field for causing the are between saidelectrodes to rotate along said gap, gas duct means for passing the gasto be ionized through said gap, said duct means comprising coolingchannels along said electrodes for maintaining their working temperatureby heat exchange with substantially all of the gas flow at a value belowthe melting point of the electrodes and above l000 C.

3. In a plasma gun according to claim 1, said two elec- 3 trodes havingcylindrical shapes of substantially the same diameters and being axiallyspaced from each other to form said annular arc gap axially between eachother.

4. In a plasma gun according to claim 3, said magnet means comprisingtwo magnet coils coaxially surrounding said respective two electrodesand axially spaced from each other at said annular gap.

5. In a plasma gun according to claim 1, said main material of saidelectrodes being chosen from the group consisting of high-temperatureresistant and corrosion-resistant steels, tungsten, molybdenum, tantalumand high-temperature alloys thereof.

6. In a plasma gun according to claim 1, said main material of saidelectrodes being chosen from the group consisting of electricallyconducting silicides, carbides and borides.

7. In a plasma gun according to claim 1, said added substances of lowwork function being chosen from the 8 group consisting of ThO K 0, BaO,'SrO, U0 UC, LaBs.

8. In a plasma gun according to claim 1, said added substances amountingmaximally to 20% by weight of said main substance.

9. A plasma gun, comprising two coaxial annular electrodes spaced fromeach other and forming an annular arc gap between each other, saidelectrodes consisting of electroconductive refractory main materialhaving a melting point above 1200 C. and an addition of substance havinga work function within 0.15 to 0.7 of that of said main material, saidaddition being at most 20% by weight of said main substance,alternating-current supply means attached to said respective electrodes,magnet coil means surrounding said electrodes to produce in said annulargap a transverse field for causing the are between said electrodes torotate along said gap, gas duct means for passing gas to be ionizedthrough said gap, said duct means comprising cooling channels along saidelectrodes for maintaining their working temperature by heat exchangewith substantially all of the gas flow at a value below the meltingpoint of the electrodes and above 1000 C.

10. A plasma gun according to claim 1, comprising direct-current supplymeans connected to said respective electrodes, and only the cathodicelectrode containing said addition of low work function.

11. In a plasma gun according to claim 10, said gas duct means formingseparate cooling channels for the cathodic and anodic electrodesrespectively.

12. In a plasma gun according to claim 1, said two electrodes beinghollow-cylindrical and axially spaced from each other to form saidannular arc gap axially between each other, said gas duct meanscomprising gas inlets near the respective electrode ends remote fromsaid gap so that the gas flows from said ends along the inner cylinderwalls of said electrodes toward the intermediate arc gap.

13. In a plasma gun according to claim 12, said gas duct meanscomprising a tubular plasma outlet duct extending from the vicinity ofsaid annular arc gap through the interior of one of said cylindricalelectrodes in substantially coaxial relation thereto.

14. Device for heating gases comprising a pair of annular electrodesconsisting of current conducting refractory main substance having amelting point above 1200 C. and an addition of substance having a workfunction within 0.15 to 0.7 of that of said main substance, means forproducing a magnetic field to rotate an arc between said annularelectrodes and means for maintaining an adjusted operating temperatureof said electrodes at a value below said melting point and above 1000C., said last mentioned means comprising structure for directingsubstantially all of the gas to be heated into cooling relationship withsaid electrodes.

References Cited UNITED STATES PATENTS 3,042,830 7/1962 Orbach 315-111X3,106,631 10/1963 Eschenbach 219- 3,129,351 4/1964 Martinek 313 231.53,130,292 4/1964 Gage 313-2315 3,134,924 5/1964 Henderson 313-463,149,253 9/1964 Luebke 3l3--346 FOREIGN PATENTS 1,097,053 1/1961Germany.

OTHER REFERENCES Laiferty Boride Cathodes published in Journal ofApplied Physics, vol. 22, No. 3, March 1951, pages 299-309.

JAMES W. LAWRENCE, Primary Examiner.

STANLEY D. SCHLOSSER, GEORGE N. WESTBY, Examiners.

1. A PLASMA GUN, COMPRISING TWO COAXIAL ANNULAR ELECTRODES SPACED FROMEACH OTHER AND FORMING AN ANNULAR ARC GAP BETWEEN EACH OTHER, SAIDELECTRODES CONSISTING OF AN ELECTROCONDUCTIVE REFRACTORY MAIN MATERIALHAVING A MELTING POINT ABOVE 1200*C., AND AT LEAST ONE OF SAIDELECTRODES CONTAINING AN ADDITION OF SUBSTANCE HAVING A WORK FUNCTIONWITHIN 0.15 TO 0.7 THAT OF SAID MAIN MATERIAL, MAGNET MEANS HAVINGTRANSVERSE FIELD IN SAID ANNULAR GAP FOR CAUSING THE ARC BETWEEN SAIDELECTRODES TO ROTATE ALONG SAID GAP, GAS DUCT MANS FOR PASSING THE GASTO BE IONIZED THROUGH SAID GAP, SAID DUCT MEANS COMPRISING COOLINGCHANNELS ALONG SAID ELECTRODES FOR MAINTAINING THEIR WORKING TEMPERATUREBY HEAT EXCHANGE WITH SUBSTANTIALLY ALL OF THE GAS FLOW AT A VALUE BELOWTHE MELTING POINT OF THE ELECTRODES AND ABOVE 100*C.